1. Field of the Invention
The invention generally relates to LED-based light sources. More particularly, the invention relates to controllable color light sources which can be controlled in terms of brightness and/or color shade.
2. Description of Related Art
LED lighting sources are widely used to provide energy-saving and flexible lighting devices. In contrast to gas discharge lamps, light sources with LEDs are distinguished by the fact that they are easier to dim, among other things. Also, with a combination of differently colored LEDs virtually any color shade can be adjusted.
LEDs are moreover distinguished by the fact that they generally have small light-emitting faces of high luminance. However, depending on the application, a larger light-emitting surface or a diffuse illumination might be desired. For this purpose, it is known from the prior art to employ light guides which distribute the light generated by the LEDs. Such an LED lighting device is known, for example, from WO 2012/130497 A1. According to this prior art, an LED lighting device is described that comprises two separate light guides which are arranged so that a respective end face of one light guide faces an end face of the other light guide, and so that an intermediate region is defined in which in the operating state the radiation of at least one LED is injected into the light guides via the end faces arranged adjacent to the intermediate region and is guided in the light guides, wherein the outer peripheral surface regions of the light guides include means for emitting radiation, which reflect the radiation guided in the light guides into the light guides so that in the operating state the radiation passes through the light guides and is laterally emitted from the light guides.
With such an arrangement, a linear illumination profile is obtained with an opening angle which may correspond to a fluorescent lamp with reflector.
The LED and/or the LED chip has an electronic evaluation and control unit associated therewith, which determines the color location and/or brightness, and in response thereto the LED or the LED chip is controlled so that specific operating conditions of the LED are adjusted. By selectively controlling the elements of the chip, mixing of the colors of the emitted radiation is achieved. Thus, the LED and therefore the lighting device is capable of emitting light of any desired color. However, the color location may vary depending on environmental conditions and on the age of the LEDs. This variation cannot be determined, and therefore the sensor is intended for measuring the intensity and color location of the radiation. By taking into account these values in the controlling of the LED or LED chip, it is possible to achieve a reproducible and permanently stable color location and reproducible and permanently stable intensity of the radiation emitted by the lighting device.
Now, starting from WO 2012/130497 A1, it would be desirable to be able to quickly adjust exact color locations and a desired change in the color location. If a light sensor is used which is integrated on an LED chip, as proposed in WO 2012/130497 A1, there is the problem that the light captured by the sensor may have a different hue than the emitted light, for example if due to the arrangement different proportions of the emitted light of the LED reach the sensor. On the other hand, if the sensor in arranged in the illuminated room, this would be annoying for structural and aesthetic reasons.